This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-263797 filed Aug. 31, 2000.
The present invention relates to an obstacle detecting system, which detects obstacles against a vehicle by the use of an ultrasonic wave.
Obstacle detecting systems for vehicles use an ultrasonic vibrator, which transmits an ultrasonic wave and receives the ultrasonic wave reflected by an obstacle. The ultrasonic vibrator has a casing, a piezoelectric device provided in the casing, and a vibration plate made of an aluminum film and provided in the opening of the casing.
The piezoelectric vibrator vibrates at a frequency of the drive voltage applied thereto. The vibration of the piezoelectric vibrator is transmitted to the vibration plate, so that the casing resonates with the vibration. Thus an ultrasonic wave is transmitted from the vibration plate. The casing continues to resonate by inertia vibration after the piezoelectric device stops the vibration, resulting in reverberating vibration. Thus, the ultrasonic vibrator transmits the ultrasonic wave in response to the vibration of the piezoelectric device and the reverberating vibration.
The ultrasonic vibrator is mounted on a vehicle in such a manner that the vibration plate is exposed to the exterior of the vehicle. As a result, snow tends to stick to or accumulate on the vibration plate of the ultrasonic vibrator in winter, thereby increasing the weight of the vibration plate. The casing therefore continues the reverberating vibration for a longer period at a frequency, which is slightly deviated from its resonant frequency.
Under this condition, the ultrasonic vibrator is likely to continue to transmit the ultrasonic wave even after starting to receive a reflected ultrasonic wave. It is difficult to distinguish whether the output signal of the ultrasonic vibrator results from the reflected wave or the reverberating vibration. Thus, the obstacle detection system tends to detect that an obstacle exists close to the vehicle although it exists remote from the vehicle.
It is an object of the present invention to provide an obstacle detecting system, which is capable of detecting and notifying accumulation of snow on an ultrasonic vibrator.
According to the present invention, an obstacle detecting system for vehicles comprises an ultrasonic vibrator having a vibration plate and mounted on a vehicle to transmit and receive an ultrasonic wave, and an obstacle is detected in response to an output signal of the ultrasonic vibrator. The system checks whether the vibration plate has accumulation of snow based on a period of reverberating vibration of the vibration plate, and notifies an abnormality of the ultrasonic vibrator arising from the accumulation of snow.
Preferably, the system executes a snow accumulation check operation only when the temperature around the ultrasonic vibrator is below a predetermined temperature corresponding to snowfall. The system calculates a ratio of the period of reverberating vibration of the ultrasonic vibrator relative to a predetermined time period following an ultrasonic wave transmission, and determines the accumulation of snow when the calculated ratio exceeds a predetermined time ratio.